Serious and persistent social-emotional and behavior problems in children under three years of age are well documented by clinical observations and empirical research. However, little is known about the prevalence, pattern and course of psychopathology in infants and toddlers. Recent developments in public policy and legislation mandate epidemiologic research on children's mental health and offer new opportunities to provide proven intervention and prevention services to eligible infants and toddlers. Valid and reliable instruments for assessing social-emotional and behavior problems are essential to achieve these research and service provision goals. Yet, no validated measure of social-emotional problems and competencies exists for 1- and 2- year old children. The primary goal of this project is to examine the prevalence, developmental patterns and correlates of psychopathology in 1- to 3-year old children. To achieve this goal, we must first validate and refine the Infant-Toddler Social & Emotional Assessment (ITSEA), a questionnaire that will be employed to assess social-emotional problems and competencies in 1- and 2- year old children. A representative community sample of 1,200 parents with 1- and 2- year old children will be recruited from the birth records at Yale-New Haven Hospital. In addition, parents of 240 1- and 2- year-old children referred for possible behavioral, developmental and/or social-emotional problems will be recruited form the South Central Region of the Connecticut Birth to Three System. To examine developmental patterns and correlates of social-emotional problems and competencies, all parents will complete measures of child behavior and temperament, family functioning, parenting stress, and parental affective symptoms. Three smaller substudies will examine the specific psychometric properties of the ITSEA (i.e., test-retest, cross-method, and inter-rater reliabilities), as well as the validity of the competence scales. All children participating in the 3 smaller studies will receive a developmental evaluation. Finally to evaluate the stability of and risk-factors associated with social-emotional problems and delays in competence, all children will be reassessed in a 1-year follow-up.